Protecting the identity is an act of securing one's control, and hence, power, and sense of survival fitness. Furthermore, groups sharing common identity strengthen the feeling of kinship, and hence again, stronger control. This is an evolutionary psychology interpretation (in which this topic has been discussed to great depths).

Notice that below a puberty it's much harder to find polarized arguments like the "wars" we talk about. One may argue that mental development is one reason, but another factor, closely related to development, is that at this age the brain is still highly malleable, and the identity is weak. A kid could argue vehemently about a particular idea (note this is different from preference, as in food). It is entirely plausible that if you get their parents and friends to influence them you could turn the entire idea around in a short time.

Not so with adults. They'd simply leave the premises and find people who agree.

That isn't to say you are wrong -- but that what you seem to think is an entirely different idea from what pg wrote is actually quite similar.

That said, your observations are not contradictory. If clashes and intolerance arise naturally because of self-identification with religious positions, how much more convenient for a aggressive ruler to exploit that as a pretext for war. The same has been done for nationalism, for at least a few centuries.